Among the easy-open cans used for canned drinks, general canned food, etc., there are the tear-off types where a tab consisting of part or all of the container lid is torn off by pulling and separated from the can body and the stay-on tab types where the tab remains attached to the can body. In these easy-open cans, coated aluminum sheet or steel sheet is used as the can opening material. This is punched out into the basic lid shape, then is placed on a flat bottom die and pressed down on by a top die on which is provided a sharp sectional edge scoring blade shaped to the contour of the tab so as to form a tab shaped tear-along groove with a V-sectional shape. To facilitate can opening, it was necessary to press-form the tear-along groove by the scoring blade so as to reach approximately ½ to ⅔ of the thickness of the sheet before processing. If the depth of the tear-along groove was too shallow, the ease of can opening would be poor, while if too deep, the strength would be insufficient and there would be problems in transport such as opening of the can due to even small external impact.
The can-opening materials are being made extremely thin due to the demands for ease of can opening. Therefore, considerable precision is being demanded for the scoring tools as well, and therefore, the problem has arisen of a significantly reduced tool life. To deal with this problem, measures have been devised to extend tool life such as with the “process for formation of a tear-off type tab for a can composed of forming a thin upward facing connection piece between the portions around the tab (tear-along bead groove) and the can body, then pushing down the tab so that the connection piece is bent at its intermediate portion to form a tear-along groove” such as in Japanese Unexamined Patent Publication (Kokai) No. 55-70434 or Japanese Unexamined Patent Publication (Kokai) No. 57-175034. Further, a can has been commercialized which is given repair coating for preventing the occurrence of rust at the portions of the metal surface exposed by the cutting of the surface treatment film by the processing to form the tear-along groove, but this repair coating, as in the main coating work, requires a complicated baking process which takes a long time and, further, there was the problem of contamination of the global environment by the carbon dioxide released from the solvent in which the coating is mixed at the time of baking.
In current day easy-open can lids, broad use is made of materials composed of aluminum or steel sheet coated with vinyl chloride organosol of a vinyl chloride coating composition, due to the advantages of processability, corrosion resistance, preservation of the flavor and taste of the contents, and price. On the other hand, however, there has been the problem that in the technology for recycling resources, when recovering used cans and incinerating or remelting them, vinyl chloride coating compositions give off toxic dioxin. In view of this problem, research and development efforts are underway on new coating composition which can take the place of such vinyl chloride composition.
Recently, to solve the above-mentioned problems and to eliminate the needs for repair coating of the tear-along groove portions, techniques have been developed for producing easy-open can lids where the tear-along groove portions are formed by press-forming the polyester resins laminated metal sheet by the die radius of top and bottom dies. In easy-open can lids, however, there has been the problem of a considerable amount of occurrence of feathering. The term “feathering” used herein means the organic film which is left at the edge portions of the cut opening on the can body side when the easy-open can lid is opened. This is disliked in that it gives an unsanitary image in the outer appearance. This sometimes was also a problem when opening cans formed with conventional easy-open can lids, that is, coated metal sheets formed with tear-along grooves by scoring the sectional surface with a sharp edge.